Polychlorinated dibenzodioxins (PCDDs) are of no commercial value and are not intentionally manufactured, but are the by-products of the manufacture of other chlorinated chemicals, such as the chlorinated wood preservatives and herbicides, and the chlorine bleaching of wood pulp for paper. PCDDs are also produced by combustion, typically by the incineration of municipal and industrial waste, metal smelting, and by burning fossil fuels and wood. Some dioxins may be of natural origin, formed during forest fires and released to the atmosphere. Current releases of PCDDs are generally due to combustion. Although actions have been taken by governments of industrialized countries to reduce industrial emissions of PCDDs to the environment, these compounds are termed "persistent organic pollutants (POPs)," and an "environmental reservoir" of past releases remain.

PCDDs are organic compounds, chemically classified as halogenated aromatic hydrocarbons. They are structurally comprised of two benzene rings joined by oxygen atoms at the "meta" and "para" positions. There are 75 congeners in the PCDD family—2 monochlorodibenzo-p-dioxins (MCDD), 10 dichlorodibenzo-p-dioxins (DCDD), 14 trichlorodibenzo-p-dioxins (TrCDD), 22 tetrachlorodibenzo-p-dioxins (TCDD), 14 pentachlorodibenzop- dioxins (PeCDD), 10 hexachlorodibenzo-p-dioxins (HxCDD), 2 heptachlorodibenzo-p-dioxins (HpCDD), and a single octachlorodibenzo-p-dioxin (OCDD). As previously noted, except for use in environmental research, PCDDs are not deliberately manufactured.

Dibenzo-p-Dioxin Molecule

Dioxins persist for a long time in the environment because their structure is resistant to chemical or biological degradation. They are subject to a very slow breakdown by sunlight (photolysis) and can be destroyed by hydroxyl radicals in the atmosphere. But because dioxins break down so slowly, past emissions remain in the environment for years—even decades—before they diminish. This environmental reservoir of PCDDs stored in soil and sediment contributes to the current contamination of food. With the reduction in current emissions from combustion and incineration, these reservoir sources may have taken on more significance.

The physical-chemical properties of the PCCDs provide explanation for the behavior of these compounds in the environment. PCDDs have relatively low vapor pressures and high octanol/water partition coefficient (Kow) values which means they are not very mobile in the environment, and tend to sorb onto soil and sediments. The high Kow values also mean that PCDDs will move easily into the fatty tissues of grazing animals, cows and sheep, that incidentally ingest them in the soil adhering to grass. PCDDs bioaccumulate and biomagnify in animal fat. Similarly, when humans consume these animals, the dioxins then accumulate in human fatty tissues. Because dioxins also persist in the body for years, recent significant reductions in dioxin emissions into the air are unlikely to reduce overall human health risks to the general population in the near term. Limited exposure to dioxins also might result from breathing air containing trace amounts of dioxins, inadvertently ingesting soil containing dioxins, and absorbing minute levels of dioxins present in the soil through the skin.

The most toxic of the PCDDs is the 2,3,7,8-TCCD congener. Other PCDDs with chlorine in the 2,3,7,8 positions appear to produce similar adverse health effects, but at higher exposure concentrations. While this web page specifically addresses PCDDs, it is important to note that structurally similar compounds such as chlorinated dibenzofurans (PCDFs) and coplanar polychlorinated biphenyl congeners are often found with PCDDs. Several of their congeners have similar toxic properties and are referred to as "dioxin like" chemicals with "dioxin like" effects.

2,3,7,8- Dibenzo-p-Dioxin Molecule

When calculating human exposures, dioxins are measured in picograms—that is, trillionths (0.000000000001) of a gram. Highly sophisticated (and costly) measurement techniques and technologies are required to test for the presence of the 2,3,7,8 substituted dioxin congeners identified as having the most toxic effects.

According to EPA, between 1987 and 2000, there was an approximately 90% reduction in the release of dioxin-like compounds (includes PCDDs, PCDFs, and select PCBs) to the circulating environment of the United States from all known sources combined. This reduction in environmental releases of dioxin-like compounds is attributable to source-specific regulations (Clean Water and Clean Air Acts), improvements in source technology, and advancements in the pollution control technologies specific to controlling dioxin discharges and releases.

Remediation technologies for the cleanup of dioxin-contaminated soils and sediments are still being developed, and many of the accepted techniques rely on thermal destruction, though physical, chemical, and biological technologies show promise.